The present invention relates to an electromagnetic bearing apparatus having five degrees of freedom for use in a turbo molecular pump, a spindle of a working machine or the like.
FIG. 1 is a view showing the general structure of a five degrees of freedom control type magnetic bearing of conventional construction. In FIG. 1, 1 is an axis position sensor for detecting the position of a rotating body 2 in the axis direction, 3 is a target corresponding to the axis position sensor 1 mounted on the rotating body 2, 4 is a motor for rotating the rotating body 2, 5 is an armature disc mounted on the rotating body 2, 6 is an axis direction electromagnet for providing an axis direction controlling force to the armature disc 5, 7 and 8 are radial direction magnetic bearings, and 9 and 10 are radial direction position sensors.
FIG. 2 is an embodiment of a prior art control system for the five degrees of freedom control type magnetic bearing arranged as shown in FIG. 1. The arrangement of the control system is disclosed in French Pat. No. 2149644 in which the translating movement parallel with the rotating axis of the rotating body and the rotating movement with respect to the center of mass of the rotating body are controlled individually. In the drawing, 11 is an adder for a pair of radial direction position sensors X.sub.1 and X.sub.1 ' or X.sub.2 and X.sub.2 ' and 12 is an adder for carrying out the adding operation for the output from the adders 11. The output from the adder 12 is indicative of the translating movement in the X axis direction and is applied to a phase advance compensating circuit 13. The output from the phase advance compensating circuit 13 is applied to adders 14 and 19 whose outputs control a power amplifier 29 by which electromagnet coils A.sub.1, A.sub.1 ', A.sub.2 and A.sub.2 ' are driven. In a similar way, a control device for restricting the translating movement in the Y direction comprises adders 20 and 21, a phase advance compensating circuit 22, adders 23 and 28 and the power amplifier 29 to control the power supplied to electromagnet coils B.sub.1, B.sub.1 ', B.sub.2 and B.sub.2 '.
A signal component of the movement around the center of mass of the rotating body is obtained by adding the output of an inverter 15 to the output of the adder 11 for the radial direction detectors X.sub.2 and X.sub.2 ' by the use of an adder 16. The output of the adder 16 is applied to a wide band phase advance compensating circuit 17 whose output drives the electromagnet coil A.sub.1 or A.sub.1 ' and the electromagnet coil A.sub.2 or A.sub.2 ' is driven by the output of an inverter 18. The movement rotating about the Y axis at the center of the mass of the rotating body is restricted by the control device mentioned above. In a similar way, the movement control around the X axis is carried out by the control device composed of the adder 20, an inverter 24, a wide band phase advance compensating circuit 26, an inverter 27 and the power amplifier 29. The electromagnetic coil B.sub.1 or B.sub.1 ' is driven by the output of the power amplifier 29 and the electromagnetic coil B.sub.2 or B.sub.2 ' is driven by the output of the inverter 27 whereby to attain the desired control.
In order to carry out the restriction control in the Z axis direction, that is, the thrust direction of the rotating body, the signals of the axis direction position detectors Z.sub.1 and Z.sub.2 are applied to an adder 30 and the control signal corresponding to the signal is produced by a phase advance compensating circuit 31. An electromagnet coil C.sub.2 is driven by controlling a power amplifier 29' in accordance with the control signal mentioned above, and the output of the phase advance compensating circuit 31 is applied to the inverter 32. The power amplifier 29' is controlled by the output signal to drive the electromagnet coil C.sub.1. As a result, the restriction control along the Z axis is attained.
The meanings of the symbols X.sub.1, X.sub.1 ', . . . A.sub.1, A.sub.1 ' . . . used for explaining the control block diagram shown in FIG. 2 are illustrated in FIG. 3. In FIG. 3, 33 is a rotating body, P.sub.1, P.sub.2 are radial direction magnetic bearings, and P.sub.3 is an axis direction magnetic bearing. A.sub.1, A.sub.1 ' designate the mounting positions of vertical direction electromagnet coils for the radial direction magnetic bearing and B.sub.1 and B.sub.1 ' designate the mounting positions of horizontal direction electromagnet coils for the radial direction magnetic bearing. C.sub.1 and C.sub.2 designate the mounting positions of electromagnet coils for the axis direction magnetic bearing P.sub.3. In FIG. 3, the directions of the arrows indicate the direction of electromagnetic force. X.sub.1 and X.sub.1 ' form a pair of position detectors which are structural members of the bearing P.sub.1 and placed in the vertical direction. Y.sub.1 and Y.sub.1 ' form a pair of position detectors which are placed in the horizontal direction. Similarly, X.sub.2 and X.sub.2 ', and Y.sub.2 and Y.sub.2 ' are pairs of position detectors by which the bearing P.sub.2 is arranged and Z.sub.1 and Z.sub.2 is a pair of position detectors by which the bearing P.sub.3 is arranged.
With the arrangement of the control block shown in FIG. 2, it is possible to control three translating movements other than the movements around the rotating axis of the rotating body and two rotating movements around the center of the mass. However, when the precession and the nutation occurs in the rotating body due to a gyro effect, the aforedescribed arrangement is not so effective as to control it. The reasons for this are as follows. For example, when the rotating motion around the X axis occurs due to the influence of the gyro effect during the high speed rotation of the rotating body 33, the rotating body 33 starts to rotate around the Y axis. However, in the control system shown in FIG. 2, no control for suppressing this effect is taken into consideration.